(a) Field of the Invention
The present invention relates to a process or a method for preparing alloys and eventually alloyed negative electrodes. The invention also relates to new industrial products, the alloys and alloyed negative electrodes thereby obtained as well as a device making use of such negative electrodes. More specifically, the present invention is directed to electrochemical generators having high densities of energy, which generally include lithium, and is more particularly directed to alloyed negative electrodes, i.e. where lithium is present in a host structure, which is generally metallic, whose activity is lower than that of metallic lithium.
(b) Description of Prior Art
In most of the generators, utilizing alkali metals with organic, liquid or polymeric electrolytes, it has been observed that lithium has a tendency to form passivating films. The formation of such films, which is discussed in detail in J. P. Gabano, "Lithium Batteries", Acad. Press, N.Y. 1983, does not generally prevent the necessary ionic exchanges needed for the discharge of the lithium electrode because these films are generally thin and ionic conductors. This is particularly the case of non-rechargeable batteries. However, it has been realized that such films can electronically insulate an important fraction of the lithium when the latter is present in particulate form or has been electrochemically redeposited. One then observes an important decrease of the utilization of the negative electrode, which decrease must then be compensated by an excess capacity of the negative electrode. This problem is substantially reduced in the case of rechargeable systems by using lithium alloys having a high rate of diffusion of lithium and which are not thermodynamically favorable to the electrochemical depositing of pure metallic lithium during recharge. This will result in preventing the formation of dendrites of lithium and the electrical insulation of freshly deposited lithium.
However, these alloys are generally hard and brittle intermetallic compounds which are generally prepared by pyrometallurgy and are consequently hard to convert into thin shapes such as by lamination. They are therefore used more often in particulate forms, pressed with a binder, as fritted substances or they can be prepared by electrochemical means, such as in the form of accumulators mounted in discharged state. These various processes designed for liquid electrolytes are not well adapted to polymeric electrolytes which make use of large surfaces and thicknesses of the order of some tens of microns.
A partial solution to these problems has been proposed in the case of polymeric electrolytes formed of complex made of polyethers combined with an alkali salt, by utilizing composite electrodes where the alloy is dispersed in particulate form in a polymeric electrolyte which also acts as a binder between the particles, and in the presence of a carbon additive which has been added to improve the electronic and ionic exchanges at the interfaces. References will particularly be made to U.S. application Ser. No. 430,696 now U.S. Pat. No. 4,517,265 in the name of Hydro-Quebec, filed Sept. 30, 1982. This solution enables to prepare thin electrodes with large surfaces which are particularly well adapted to thin film generators operating with polymeric electrolytes. However, it has been observed that such electrodes can present in certain cases, particularly at operating temperatures higher than 80.degree. C., a progressive decrease of utilization of the alloys during successive cycles. Moreover, such electrodes remain costly to prepare in view of the high cost of the starting alloys generally prepared by pyrometallurgy, for examples about $500/lb for LiAl. It should also be remembered that these alloys should then be crushed, screened and mounted in the form of electrodes under strictly inert atmosphere.